Return of the Megatsunami

(reposted, with some modifications, from ye olde blog)
It seems like megatsunamis are back in vogue, but this time a different culprit is in the frame. Around the turn of the millennium a lot of publicity was given to the suggestion that a 200 cubic kilometre-sized chunk of the western flank of Cumbre Vieja, an active volcano on La Palma in the Canary Islands, was poised to fall into the sea in a future eruption. Modelling suggested such a collapse would send a gigantic tsunami racing westward across the Atlantic, and the eastern seaboard of the US would be assaulted by 50 m high waves that would penetrate up to 20 km inland (for comparison, the Boxing Day 2004 tsunami produced waves 20-30 m high which penetrated 2-3 km inland on the Sumatran coast, a mere 160 km away from the epicentre).
This apocalyptic scenario was put forward by Bill McGuire and his colleagues at the Benfield Hazard Research Centre. McGuire has become the go-to guy when you want a geological scare-quote for your breathless documentary about how we’re all doomed, whether by megatsunami, supervolcano, or some other hyperbolically titled geological phenomenon. In my old department in Southampton, some people who have studied past collapse events in the Canaries think that the risk is being a tad…overhyped (see also here for the Benfield team’s counterargument), because you seem to get a series of much smaller landslides rather than one big one which will Wipe Out the Whole of Western Civilisation.
At the end of 2006 Phil called our attention to an article in the New York Times (free registration required) which highlights another potential source for really big waves – oceanic asteroid impacts, which a group of scientists led by Dallas Abbott have proposed as the source of some interesting wedge-shaped deposits, referred to as ‘chevrons’, you find on some shorelines, particularly around the Indian Ocean. Firing up Google Earth, we can find them on the West Australian coast (the line on this and the following images is 5 km long:

and here are the stars of the NY Times article, in Fenabosy bay, Madagascar:

and some similar structures I’ve spotted on the coast of Mozambique (south-east Africa):

From a distance the chevrons resemble large dunes, but when you look closer, certain features don’t seem to fit. Studies of the Australian chevrons (see here and here) suggest that although they’re largely made up of sand, they also contain shell debris and cobbles (clasts of rock about 10-20cm in diameter), and sometimes are associated with much larger boulders, which appear to have been transported a considerable distance, and are sometimes even imbricated, indicating powerful current flows moving inland from the shore.

These observations are consistent with the chevrons being formed by a violent surge of water – a storm surge or a tsunami – sweeping the shells and other marine material inland. The strength of current required to transport the large boulders, and the distances inland (several km) and height above present sea level (10s of m, in some cases around 100 m) this material now crops out, have led some people to favour a tsunami origin.
So where do the asteroids come in? Look at this animation of the propagation of the Boxing Day 2004 tsunami (for those with lots of bandwidth, go here for the high resolution Quicktime version), and compare the wave directions radiating away from the Sunda Trench with the orientation of the chevrons found around the Indian Ocean. It’s clear that the former cannot cause the latter, so the chevron-causing tsunamis were not generated by subduction zone earthquakes (all of the other plate boundaries in the Indian Ocean are spreading ridges). Even if the orientation wasn’t wrong, the size of wave required to form structures on the scale of the chevrons dwarfs even the tsunami generated by the magnitude 9.2 earthquake two years ago.
Hence we need another source, and Abbott and the other members of the ‘Holocene Impact Working Group’ reckon that source is an asteroid impact, specifically the one which formed the 18-mile wide Burckle Crater, located in the centre of the Indian Ocean (31¬?S, 61¬?E). To support their hypothesis, they cite analysis of some of the marine fossils found in the Madagascar chevrons, which are fused with metallic flecks enriched with elements common in chondritic asteroids but rare in the Earth’s crust (as Lab Lemming explains rather better than the NY Times article).
This is not the only recent large impact Abbott and her colleagues have proposed – you can find chevrons on the eastern coast of Australia and southern New Zealand which they’ve linked to a crater on the New Zealand continental shelf. That’s at least two major impacts in the last 10,000 years, which is a bit at odds with the current estimated frequency calculated from astronomical surveys of near-earth asteroids.
In the midst of this contradiction, then, it is worth noting that the evidence is still slightly ambiguous. From the information I can find online there is reasonable evidence that at least some of the chevrons are caused by tsunamis, but the orientation of many of the Australian chevrons seems a little too north-south to be obviously traced back to Burckle Crater, which is almost directly to the east; and in the second of the two papers I linked to above, carbon dating of coral and shell debris in the chevrons suggests that together they record at least three events, roughly 700, 2000 and 6000 years ago – Burckle Crater is thought to be about 5000 years old – rather than one big event, although this may be complicated by reworking of older debris. Perhaps, then, we are looking at evidence for a number of smaller impacts, perhaps mixed in with submarine landslides, rather than a single big one forming all the chevrons. Of course, more common very large tsunami events is only marginally less worrying than rarer mega-tsunamis, but such an interpretation would probably be more consistent with the astronomical data – there’s probably more uncertainty in impact rates of smaller asteroids, as they’re harder to spot (while we’re on that subject, it seems that estimates of the size of tsunami generated by an impact of a given size vary widely, which introduces additional uncertainties). However, only time, and some careful dating work, will tell.

Comments (10)

Something that has bothered me for a long time about the asteroid impact estimates is that, to riff a little on A Prairie Home Companion, “everyone’s children are below average”.
We count young impact craters and come up with one number for impact rates. We count known asteroids on potential collision courses with Earth and we come up with a very different (a much lower) number.
I’m not the only one to notice this discrepancy. A 2005 paper, Earth in the Cosmic Shooting Gallery (PDF) (Asher, D. J.; Bailey, M.; Emel’Yanenko, V.; Napier, W., The Observatory, Vol 125, p 319-322), with the summary “The terrestrial impact rate appears to be substantially higher than current near-Earth object population models imply, consistent with a signiÔ¨Åcant unseen cometary contribution to the terrestrial impact hazard.” makes the same point in more analytic terms.

One thing I haven’t seen discussed is the number of chevrons– you’ll see a dozen or more of them in those images. Do you get more than one chevron per impact? If not, doesn’t it seem implausible that there are so many, all facing the same way?
They imply that either you get multiple chevrons in a single event, or a recurring event that generates them. The latter precludes asteroid impacts, I think.

I’m not great shakes as an amateur astronomer, but as I understand it, there was a finite number of asteroids to begin with. When the solar system first formed, there was an early heavy bombardment of asteroid strikes on all planets – not just Earth. This left considerably fewer asteroids out there. Hence the discrepancy between the past impact rate and the current number of asteroids.
And “over-hyped” is being kind. See the entry for Feb 6th

I said “young impact craters”. Note the word “young”. The discrepancy cannot be resolved by saying “they used to hit more often billions of years ago”. While that is very true, the problem is that they still hit much more often than can be accounted for by the known and estimated unknown asteroids in the inner solar system as predicted by current theories.
There must be some source of recent Earth impactors we haven’t found yet. The article I linked to suggests that that source is in the Oort Cloud.

I went to a talk the other week about the breakup of a large asteroid ~160 Ma, claiming that this was the source of a large population of Earth impactors (including the K-T boundary event). IIRC the parent body was pretty low albedo.